The invention relates to a process for purifying (meth)acrylic acid. More specifically, the process is a process that provides (meth)acrylic acid containing very low levels of residual aldehyde.
In the manufacture of acrylic acid by the catalytic oxidation of propylene, acrylic acid (“AA”) and oxidation by-products initially are obtained by absorbing or quenching reactor effulent gases with water to produce an aqueous AA solution before further purification. Fouling, e.g., the in situ formation of acrylic polymer on the processing equipment internals, is a major problem associated with the production of AA. Fouling can be exacerbated by impurities in the AA solution.
Aldehydes are an undesirable impurity, as they can cause fouling in the manufacturing process, and as they cause problems when present in polymer grade AA. The presence of these aldehyde impurities and their hydrates in aqueous media is detrimental to downstream acrylic acid processing for multiple reasons including: (1) the demonstrated ability of these contaminants to destabilize AA by initiating acrylic acid free radical polymerizations, (2) active participation in downstream process column fouling by self polymerization, co-polymerization with other reactive monomers, and reaction in some instances with process inhibitors, and (3) negatively impacting product quality by contamination of the final acrylic acid product if not fully removed beforehand. The conversion of these reactive aldehyde impurities to non-reactive heavies should ideally be performed as early in the acrylic acid purification process as possible.
In currently commercial AA processes, it is common to find a crystallizer and/or several distillation columns, or towers. However, as taught in U.S. Pat. No. 8,242,308, each tower requires the addition of fresh polymerization inhibitor to inhibit polymer fouling.
Furthermore, conventional fractional distillation alone is not effective in reducing aldehyde impurities to necessary levels to produce pure grade acrylic acid (“PGAA”), which is useful for the application of purposely producing polymers. To obtain PGAA, less pure AA from either the extraction/distillation or direct distillation procedure must be purified beyond that achieved by conventional fractional distillation because residual impurities, particularly the aldehydes, interfere with PGAA-related end use polymerization reactions. For these applications, aldehyde levels individually must be below about ten parts per million (ppm), more preferably below five ppm, and most preferably below one ppm. PGAA having these aldehyde levels is useful in producing, for example, superabsorbent polymers and polymers efficient as dispersants for oil well drilling muds and as flocculating agents.
One known method of producing PGAA applies aldehyde scavengers to dry AA in order to reduce aldehyde levels. Specifically, U.S. Pat. No. 5,759,358 produces PGAA by sequentially applying, during the final stages of distillation, selected groups of amines to reduce acrolein and furfural.
It would be desirable to be able to reduce aldehyde content early in the manufacturing process. Benefits of early aldehyde removal include, but are not limited to, reduced costs associated with fouling-related column shut-down/clean-out and associated lost AA production.